Kettles are well known devices for boiling water in the home and office, generally for use in preparing food and drinks for human consumption. In most cases, kettles are powered by electrical energy, since it is a clean and convenient power source. Electric kettles are currently designed to give maximum heating rates and efficiency, as well as having an appealing aesthetic appearance. The most usual way to apply electrical energy to heat water in a kettle, is via one or more resistive heating elements immersed in the water.
However, the use of heating elements to rapidly heat water gives rise to high temperature gradients between the element and the water in contact with the heating element. This, together with the fact that the heating element provides multiple nucleation points for bubble formation, leads to the creation of many small bubbles of steam, which then collapse in the bulk water to produce a noise.
Furthermore, the extreme temperature gradient present at the heating element surface results in the precipitation of minerals and other chemicals dissolved in the water. This results in both the build up of precipitates around the heating element, which reduces the heating efficiency of the heating element, and across the bottom of the kettle in general, which can result in these precipitates finding their way into the food and drinks prepared using the heated water. These are both undesirable traits in kettles according to the conventional art.
A technique known as ‘ohmic heating’ can also be used to heat electrically conductive fluids such as water. Ohmic heating of water allows rapid, highly efficient heating of the water. Ohmic heating provides a more uniform heating of a fluid, and also has the advantage that none of the surfaces of the heating apparatus become much hotter than the fluid. Thus the use of ohmic heating does not result in the formation of bubbles of steam throughout the majority of the heating cycle with the result that the operation of an ohmically heating kettle is virtually silent, until the full bulk of the water boils, at which time the heating operation is complete.
Arrangements are known where current is passed through a liquid to cause it to heat. In particular, document GB 2268671 discloses an ohmic heater for heating a fluid by passing electricity through it.